1. Field of the Invention
The present invention relates to method and apparatus for controlling a moving speed of a moving body.
2. Discussion of Related Art
In a conventional ink-jet printer or any other conventional printer of a type wherein printing is effected on a sheet of paper while a printing head is moved, a moving speed of a carriage which carries the printing head is controlled to assure accurate positioning of a printed image.
Generally, the ink-jet printer uses a DC motor for driving the carriage. To control the moving speed of the carriage, the operating speed the DC motor (hereinafter referred to as “CR motor”) is controlled by a voltage signal (hereinafter referred to as “PWM signal”) which is regulated by pulse width modulation (PWM).
When a printing operation is initiated, the carriage is accelerated from the present position, that is, from a zero position or movement-start position to a printing-start position, as indicated in FIG. 18A, so that the moving speed of the carriage is raised to a predetermined desired or target value when the carriage reaches the printing-start position. During the following movement of the carriage from the printing-start position, the moving speed of the carriage is controlled to be held constant at the target speed.
In an accelerating region between the zero position and the printing-start position, the moving speed of the CR motor is controlled in an open-loop manner, so as to gradually rise, by gradually increasing the duty ratio of the PWM signal, which is a ratio of the ON time to the period of the signal (a sum of the ON and OFF times).
When the carriage has reached the printing-start position, the mode of control of the operating speed of the CR motor is changed from the open-loop control to a feedback control (PID control) in which the operating speed of the CR motor is controlled so as to eliminate or zero a speed control error between the actual moving speed and the target value of the carriage.
Recently, there has been a requirement for shortening the length of the accelerating region, that is, the distance of the accelerating movement of the carriage, in order to meet a need of reducing the size of the printer. To permit a rise of the moving speed of the carriage to the target value within the relatively short accelerating region, the duty ratio of the PWM signal must be increased at a high rate in the open-loop control of the operating speed of the CR motor. This high rate of increase of the duty ratio may cause a risk of a considerable amount of overshoot of the moving speed of the carriage, namely, an undesirable rise of the moving speed beyond the target value, before the carriage has reached the printing-start position, as indicated in FIG. 18B. If the feedback control of the operating speed of the CR motor is initiated following the open-loop control which has caused a considerable amount of overshoot of the moving speed of the carriage beyond the target value, the moving speed of the carriage suffers from a vibration or oscillation in an initial portion of a constant-speed region (printing region) following the accelerating region. It takes some time until the vibration has been fully attenuated, that is, until the moving speed of the carriage can be controlled to coincide with the target value. Accordingly, there is a risk that the actual printing position deviates from the nominal printing position in the initial portion of the constant-speed or printing region.
The amount of overshoot of the moving speed of the carriage tends to be large, particularly when the weight of the printing head is made small as a result of consumption of an ink accommodated in the printing head. In this case, it takes a considerably long time until the moving speed of the carriage coincides with the target value, leading to a risk of considerable deterioration of the printing performance of the ink-jet printer.